The present invention relates to an optical system of an optical pick-up employed in an optical disc device which is used for recording data to and/or reproducing data from a plurality of types of optical discs having different thicknesses of cover layers.
Various types of optical discs on which digital information is recorded at various densities and which have different thicknesses of cover layers have been widely used. For example, a DVD (digital versatile disc) has a recording density higher than that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
In the optical system of optical pick-up, the amount of spherical aberration changes depending on the thickness of a cover layer of the optical disc being used. Therefore, when the optical disc is changed to another having different thickness of cover layer and different recording density, a numerical aperture (NA) of light used for recording/reproducing has to be changed in response to the difference of the recording density while the spherical aberration caused by the difference of the thickness of cover layer has to be corrected.
For example, for the optical disc having relatively high recording density, high NA is required so that a diameter of a beam spot is reduced. The diameter of the beam spot decreases as wavelength of light decreases. Therefore, in general, wavelength of 780˜830 nm is used for the CD, and wavelength of 635˜665 nm is used for the DVD. To support the CD and the DVD, recent optical disc devices employ a light source unit which can emit laser beams having different wavelengths.
Japanese Patent Provisional Publication No. 2000-81566 discloses a CD/DVD compatible objective lens (i.e., an objective lens of an optical pick-up that can be used for both of the DVD and CD). The CD/DVD compatible objective lens has a diffracting structure on one of its lens surfaces. The diffracting structure has a plurality of concentric annular steps having a function of properly converging an incident beam on a data recording layer of an optical disc regardless of the thickness of a cover layer of the optical disc.
That is, the diffracting structure has wavelength dependence such that the spherical aberration changes depending on the wavelength of an incident beam. Consequently, the CD/DVD compatible objective lens is capable of supporting various types of optical discs having different thicknesses of the cover layers.
More specifically, the lens surface having the diffracting lens structure is divided into an inner region including an optical axis thereof and an outer region located outside the inner region. The diffracting structure formed within the inner region has a function of appropriately converging the beam with wavelength for the CD on the data recording layer of the CD and has a function of appropriately converging the beam with wavelength for the DVD on the data recording layer of the DVD.
The diffracting structure formed within the outer region appropriately converges the beam with wavelength for the DVD on the data recording layer of the DVD while diffusing the beam with wavelength for the CD as flare light so that the diameter of the beam spot is not reduced excessively on the data recording layer of the CD.
With the above mentioned configuration, only the beam passing through the inner region is converged on the data recording layer when the CD is used, and therefore the beam spot having relatively large diameter is formed on the data recording layer of the CD.
On the other hand, when the DVD is used, the beam passing through both of the inner region and the outer region is converged on the data recording layer of the DVD, which increases the NA. Therefore, the beam spot having reduced diameter is formed on the data recording layer of the DVD.
When the flare light, which is reflected from the data recording layer when the CD is used, is detected by a signal detecting unit provided in the optical disc device, noise may be caused on servo signals such as FE (Focus Error) signal and a TE (Tracking Error) signal.
Japanese Patent Provisional Publication No. 2002-333576 discloses an objective lens having lens surface on which a diffracting structure is formed. The lens surface is divided into an inner region and an outer region. The diffracting structure disclosed in 2002-333576 is configured such that the light incident on the outer region is diffracted into a plurality of diffraction orders of light.
With this structure, the light for the CD incident on the outer region is diffused into a wide area on the data recording layer of the CD, which reduces intensity of the flare light on the data recording layer of the CD. Therefore, the level of noise introduced on the servo signals can be reduced.
However, the advantage that the light for the CD is diffracted into a plurality of diffraction orders of light is attained only when the CD is positioned at a proper position in which a surface of the CD is perpendicular to an optical axis of the objective lens. If the CD is tilted from the proper position, the light, which is reflected from the CD and has high intensity, may be incident on the outer region.
Assuming that a tilt angle of the CD with respect to the proper position is significantly small (e.g., smaller than 1°), only a portion of the high intensity light reflected from the CD is incident on the outer region. That is, a small portion of high intensity light reflected from the CD is incident on a portion within the outer region adjacent to the inner region.
If such a phenomenon occurs, the high intensity light incident on the portion within the outer region adjacent to the inner region is hardly diffracted by the diffracting structure on the lens surface because the number of diffractive ring-shaped zones in the portion within the outer region adjacent to the inner region, on which the high intensity light is incident, is relatively small. Consequently, the high intensity light passing through the portion within the outer region adjacent to the inner region is incident on a light receiving portion of the signal detecting unit as undesired light.
The undesired light incident on the light receiving portion in the signal detecting unit causes noise on the TE signal or the FE signal, in particular, on the TE signal generated on the basis of the three-beam method.